1. Field of the Invention
The present invention is generally concerned with decisions to be arrived at in a receive subsystem of a multistate modulation-demodulation system. The present invention is more particularly concerned with a decision device for a non-linear modulation-demodulation system, typically a frequency modulation-demodulation system.
2. Description of the Prior Art
In a receive subsystem a decision device is connected on the output side of a demodulator device. The decision device determines successive symbols received at respective successive sampling times by comparing the level of the demodulated signal with predetermined thresholds.
FIGS. 1 and 2 respectively show a transmit subsystem 1 and a receive subsystem 2 together forming a frequency modulation-demodulation system with four states known as the 4FSK or four-frequency shift keying system.
Referring to FIG. 1, the transmit subsystem 1 comprises a serial-to-parallel converter 10, a digital-to-analog converter 11, a low-pass filter 12 and a voltage-controlled oscillator 13. If the link between the subsystems 1 and 2 is of a radio type, an output of the transmit subsystem 1 and an input of the receive subsystem 2 are connected to respective antennas 14 and 20. The receive subsystem 2 comprises, connected to the antenna 20, a frequency converter stage 21 in which the signal frequency is converted to an intermediate frequency (I.F.), a band-pass filter 22, a frequency discriminator 23, a low-pass filter 24, a decision device 25 and a parallel-to-serial converter 26.
A baseband serial binary signal Sb to be transmitted is converted in the converter 10 into a signal made up of symbol words a.sub.k on two parallel binary elements. Each word a.sub.k is associated with two binary elements of the serial signal Sb and is part of a symbol word alphabet {00, 01, 10, 11}. Each word a.sub.k is converted in the digital-to-analog converter 11 into a symbol S.sub.k having one of four levels -3, -1, 1 or 3 according to the respective symbol word 00, 10, 01 or 11. The signal formed by the successive symbols S.sub.k is applied to an input of the oscillator 13 through the filter 12 which smooths the spectrum of the signal made up of the successive symbols. The oscillator 13 produces a 4FSK modulated signal with four states assuming a respective one of four frequencies FO-3Fd, FO-Fd, FO+Fd, FO+3Fd, according to the respective level -3, -1, 1 or +3 of the symbol S.sub.k. FO and Fd are respectively the nominal frequency of the oscillator 13 and a predetermined frequency difference.
The 4FSK modulated signal is transmitted by the transmit antenna 14, then received by the receive antenna 20. It is fed from the antenna 20 to the frequency converter stage 21 which converts the spectrum of the received 4FSK modulated signal into an intermediate frequency signal S.sub.FI. The signal S.sub.FI is filtered by the band-pass filter 22 in a narrow frequency band which is centered on an intermediate center frequency corresponding to the frequency FO converted. The filtered signal SF at the output of the filter 22 is fed to an input of the discriminator 23. By carrying out time differentiations the discriminator 23 discriminates between the frequencies in the filtered signal SF and delivers a signal SD whose levels are substantially equal to -3, -1, 1, 3 according to the symbols S.sub.k transmitted. This signal SD is low-pass filtered in the filter 24 to produce a baseband signal S*. In the decision device 25 the signal S* is sampled at the frequency of the transmitted symbols S.sub.k. The levels assumed by the signal S* at the sampling times are compared to thresholds in the device 25 which delivers the symbol words a.sub.k according to the result of these comparisons. By applying parallel-to-serial conversion to the symbol words a.sub.k the converter 26 delivers the original binary signal Sb transmitted.
FIG. 3 is a diagram showing the theoretical eye diagram of a baseband signal S* resulting from four-state frequency modulation and demodulation, as delivered by the low-pass filter 24 in the receive subsystem 2. In this theoretical diagram, the vertical aperture d of each of the upper eye Os, the center eye Oc and the lower eye Oi is maximal and equal to the difference between the respective levels of two adjacent symbols, i.e., d=(3-1)=1-(-1)=-1-(-3)=2. A theoretical diagram like this represents null intersymbol interference at the sampling times. In the decision device 25 the baseband signal S* delivered by the filter 24 is sampled at meaningful sampling times IE and the levels of the resulting samples are compared to three thresholds Ss, Sc and Si shown on the righthand side in FIG. 3. There are four distinct outcomes for each sample. If the level of the sample is above the upper threshold Ss, the decision device 25 delivers the sample word "11" associated with the symbol S.sub.k =3; if the sample level lies between the upper threshold Ss and the center threshold Sc, the decision device 25 produces the symbol word "01" associated with the symbol S.sub.k =1; if the sample level lies between the center threshold Sc and the lower threshold Si, the symbol word "10" is produced by the device 25; finally, for a symbol level below the threshold Si, the symbol word "00" associated with the symbol S.sub.k =-3 is produced.
In the decision device 25 of a receive subsystem of the prior art the decision thresholds Ss, Sc and Si are constant and the level of the received signal S* is varied relative to the constant decision thresholds. This variation may be the result of a "displacement" and/or an "amplification" of the baseband signal S*.
As shown in FIG. 5, the means used to vary the baseband signal S* are in the form of an amplifier 27 with automatic gain control (AGC) and an adder 28, both on the input side of the decision device 25 in the receive subsystem 2, and a comparator 29. Inputs of the comparator 29 respectively receive, at the symbol transmission frequency, the samples a.sub.k * resulting from the sampling 25a of the baseband signal S* and the corresponding symbols S.sub.k produced by the decision device 25. By comparing the signals a.sub.k * and S.sub.k the comparator produces two error signals .epsilon.1 and .epsilon.2 respectively applied to a control input of the amplifier 27 and to a second input of the adder 28. In response, the amplifier 27 amplifies the received signal S* and the adder 28 displaces the DC component in the received signal, and therefore in the baseband signal S* so that the level of each sample a.sub.k * is slaved to the level of the corresponding symbol S.sub.k. The variation of the signal S* reduces the error rate in the received binary signal Sb at the output of the parallel-to-serial converter 26.
The displacement operation of the signal S* by the adder 28 can be regarded as a simultaneous displacement or translation of the three thresholds Ss, Sc and Si.
The amplification of the signal S* by the amplifier 27 can be regarded as two respective translations of equal magnitude and opposite sign of the thresholds Ss and Si relative to the threshold Sc.
Controlling the amplitude of the signal S* according to constant predetermined thresholds Ss, Sc and Si by means of an automatic gain control amplifier 27 produces a result that is satisfactory in terms of error rate reduction provided that the upper and lower eye patterns Os and Oi in the eye diagram have apertures d which are substantially equal and symmetrical about a horizontal axis corresponding to the threshold Sc in FIG. 3. In practice, although this condition is met by linear type modulation systems, such as amplitude modulation systems, it is not met by non-linear modulation systems, typically frequency modulation systems.
As shown in the eye diagram of FIG. 4 relating to four-state frequency modulation, the upper eye pattern Os and the lower eye pattern Oi do not have the above-stated symmetry and eye dimension characteristics. FIG. 4 also shows the theoretical optimal upper and lower thresholds Ss and Si for null intersymbol interference. It can be seen that reducing the error rate to a minimal value entails a negligible increase A of the upper threshold Ss to yield a modified upper threshold SMs and a more marked decrease D of the lower threshold Si to yield a modified lower threshold SMi. This modification of the thresholds Ss and Si to yield the thresholds SMs and SMi does not result from equal and opposite translations of the respective thresholds Ss and Si relative to the center threshold Sc, as is effected by an automatic gain control amplifier 27 in the prior art.